docs: def check_version(dstore):
      """
      :param dstore: a DataStore instance
      :returns:
          a message if the stored version is different from the current version
      """
      ds_version = dstore.hdf5.attrs['engine_version']
      if ds_version != __version__:
          return (': the datastore is at version %s, but the exporter at '
                  'version %s' % (ds_version, __version__))
      else:
          return ''